In this paper, a deadlock prevention policy for flexible manufacturing systems (FMS) is proposed, which can obtain a maximally permissive liveness-enforcing Petri net supervisor while the number of ...control places is compressed. By using a vector covering approach, the sets of legal markings and first-met bad markings (FBM) are reduced to two small ones, i.e., the minimal covering set of legal markings and the minimal covered set of FBM. A maximally permissive control purpose can be achieved by designing control places such that all markings in the minimal covered set of FBM are forbidden and no marking in the minimal covering set of legal markings is forbidden. An integer linear programming problem is designed to minimize the number of control places under an assumption that a control place is associated with a P-semiflow. The resulting net has the minimal number of control places on the premise that the assumption holds, and possesses all permissive states of a plant. The only problem of the proposed method is its computational complexity that makes it inapplicable to large-scale Petri net models. An FMS example from the literature is presented to illustrate the proposed method.
Knee osteoarthritis (OA) is a major cause of disability in the elderly, however, there are few studies to estimate the global prevalence, incidence, and risk factors of knee OA.
For this study, we ...searched PUBMED, EMBASE and SCOPUS from inception to April 4, 2020, without language restriction. We identified eligible studies with information on the prevalence or incidence of knee OA in population-based observational studies and extracted data from published reports. We did random-effects meta-analysis to generate estimates. This study was registered with PROSPERO (CRD42020181035).
Out of 9570 records identified, 88 studies with 10,081,952 participants were eligible for this study. The pooled global prevalence of knee OA was 16⋅0% (95% CI, 14⋅3%-17⋅8%) in individuals aged 15 and over and was 22⋅9% (95% CI, 19⋅8%-26⋅1%) in individuals aged 40 and over. Correspondingly, there are around 654⋅1 (95% CI, 565⋅6–745⋅6) million individuals (40 years and older) with knee OA in 2020 worldwide. The pooled global incidence of knee OA was 203 per 10,000 person-years (95% CI, 106–331) in individuals aged 20 and over. Correspondingly, there are around annual 86⋅7 (95% CI, 45⋅3–141⋅3) million individuals (20 years and older) with incident knee OA in 2020 worldwide. The prevalence and incidence varied substantially between individual countries and increased with age. The ratios of prevalence and incidence in females and males were 1⋅69 (95% CI, 1⋅59–1⋅80, p<0⋅00) and 1⋅39 (95% CI, 1⋅24–1⋅56, p<0⋅00), respectively.
Our study provides the global prevalence (16⋅0% 95% CI, 14⋅3%-17⋅8%) and incidence (203 per 10,000 person-years 95% CI, 106–331) of knee OA. These findings can be used to better assess the global health burden of knee OA. Further prospective cohort studies are warranted to identify modifiable risk factors for providing effectively preventive strategies in the early stages of the disease.
This work was supported by grants from the National Natural Science Foundation of China (nos. 81772384 and 81572174).
Flying insects capable of navigating in highly cluttered natural environments can withstand in-flight collisions because of the combination of their low inertia
and the resilience of their wings
, ...exoskeletons
and muscles. Current insect-scale (less than ten centimetres long and weighing less than five grams) aerial robots
use rigid microscale actuators, which are typically fragile under external impact. Biomimetic artificial muscles
that are capable of large deformation offer a promising alternative for actuation because they can endure the stresses caused by such impacts. However, existing soft actuators
have not yet demonstrated sufficient power density to achieve lift-off, and their actuation nonlinearity and limited bandwidth create further challenges for achieving closed-loop (driven by an input control signal that is adjusted based on sensory feedback) flight control. Here we develop heavier-than-air aerial robots powered by soft artificial muscles that demonstrate open-loop (driven by a predetermined signal without feedback), passively stable (upright during flight) ascending flight as well as closed-loop, hovering flight. The robots are driven by multi-layered dielectric elastomer actuators that weigh 100 milligrams each and have a resonance frequency of 500 hertz and power density of 600 watts per kilogram. To increase the mechanical power output of the actuator and to demonstrate flight control, we present ways to overcome challenges unique to soft actuators, such as nonlinear transduction and dynamic buckling. These robots can sense and withstand collisions with surrounding obstacles and can recover from in-flight collisions by exploiting material robustness and vehicle passive stability. We also fly two micro-aerial vehicles simultaneously in a cluttered environment. They collide with the wall and each other without suffering damage. These robots rely on offboard amplifiers and an external motion-capture system to provide power to the dielectric elastomer actuators and to control their flight. Our work demonstrates how soft actuators can achieve sufficient power density and bandwidth to enable controlled flight, illustrating the potential of developing next-generation agile soft robots.
This paper deals with the enforcement of nonlinear constraints on Petri nets. A supervisory structure is proposed for a class of nonlinear constraints. In order to enforce a nonlinear constraint on a ...Petri net, we propose a transition transformation technique to replace a transition in an original net by a set of transitions. Then, a control place is designed to control the firing of these transitions, aiming to enforce the nonlinear constraint. The proposed supervisor is maximally permissive in the sense that it can make all markings in the admissible-zone reachable and all markings in the forbidden-zone unreachable. The proposed method is applicable to bounded Petri nets. Finally, a number of examples are provided to demonstrate the proposed approach.
Layered double hydroxides (LDHs) have been recognized as potent electrocatalysts for oxygen evolution reaction (OER), but are lacking in hydrogen evolution reaction (HER) activities due to the ...sluggish kinetics of water dissociation in alkaline medium. Herein, aiming to simultaneously bolster the HER and OER kinetics, a metal–organic framework (MOF) mediated topotactic transformation tactic is deployed to fabricate holey ternary CoFeNi LDHs on nickel foam, exposing polygonal mesopores with atomistic edge steps and lattice defects. The optimized catalyst requires only an external voltage of 1.49 V to afford the water splitting current density of 10 mA cm−2 apart from the superb electrolytic stability, far surpassing the benchmark Pt/C||RuO2 couple. More importantly, mechanistic investigations utilizing advanced spectroscopies in conjunction with density function theory (DFT) understandings unravel while the synergetic effect among under‐coordinated metal centers lowers the energy barrier of water dissociation, Fe‐doping enables further modulating the d‐band density of states (DOS) of Co and Ni in favor of intermediates binding, thereby promoting the intrinsic HER activity. Operando Raman studies reveal negligible structural change of the LDHs during the HER process, whereas for OER the active sites can quickly turn into oxyhydroxides in the presence of lattice defects and under‐coordinated metal centers.
A unique metal–organic‐framework‐mediated topotactic transformation strategy is deployed to fabricate holey ternary layered double hydroxides, exposing polygonal mesopores with atomistic edge steps and lattice defects. The under‐coordinated metal centers and Fe‐modulated electronic states synergistically promote the kinetics of water dissociation in hydrogen evolution reaction, ultimately leading to superb overall water splitting performance.
Although giant unilamellar vesicles (GUVs) have been extensively studied as synthetic cell-like microcompartments, their applicability as cytomimetic models is severely compromised by low levels of ...membrane permeability, low encapsulation efficiencies, and high physicochemical instability. Here, we develop an integrated cytomimetic model comprising a macromolecularly crowded interior with high sequestration efficiency and enclosed within a phospholipid membrane that is permeable to molecules below a molecular weight cutoff of ca. 4 kDa. The protocells are readily prepared by spontaneous assembly of a phospholipid membrane on the surface of preformed polynucleotide/polysaccharide coacervate microdroplets and are designated as giant coacervate vesicles (GCVs). Partial anchoring of the GCV membrane to the underlying coacervate phase results in increased robustness, lower membrane fluidity, and increased permeability compared with GUV counterparts. As a consequence, enzyme and ribozyme catalysis can be triggered in the molecularly crowded interior of the GCV but not inside the GUVs when small molecule substrates or inducers are present in the external environment. By integrating processes of membrane-mediated compartmentalization and liquid–liquid microphase separation, GCVs could offer substantial advantages as cytomimetic models, synthetic protocells, and artificial biomolecular microreactors.
Coacervate microdroplets, formed
via
liquid-liquid phase separation, have been extensively explored as a compartment model for the construction of artificial cells or organelles. In this study, ...coacervate-in-coacervate multi-compartment protocells were constructed using four polyelectrolytes, in which carboxymethyl-dextran and diethylaminoethyl-dextran were deposited on the surface of as-prepared polydiallyldimethyl ammonium/deoxyribonucleic acid coacervate microdroplets through layer-by-layer assembly. The resulting multi-compartment protocells were composed from two immiscible coacervate phases with distinct physical and chemical properties. Molecule transport experiments indicated that small molecules could diffuse between two coacervate phases and that macromolecular enzymes could be retained. Furthermore, a competitive cascade enzymatic reaction of glucose oxidase/horseradish peroxidase-catalase was performed in the multi-compartment protocells. The different enzyme organization and productions of H
2
O
2
led to a distinct polymerization of dopamine. The spatial organization of different enzymes in immiscible coacervate phases, the distinct reaction fluxes between coacervate phases, and the enzymatic cascade network led to distinguishable signal generation and product outputs. The development of this multi-compartment structure could pave the way toward the spatial organization of multi-enzyme cascades and provide new ideas for the design of organelle-containing artificial cells.
A coacervate-in-coacervate micro-architecture is constructed as a multi-compartment protocell model, in which a multi-enzyme cascade is spatially organized for competitive enzymatic reactions.
Several animal species demonstrate remarkable locomotive capabilities on land, on water, and under water. A hybrid terrestrial-aquatic robot with similar capabilities requires multimodal locomotive ...strategies that reconcile the constraints imposed by the different environments. Here we report the development of a 1.6 g quadrupedal microrobot that can walk on land, swim on water, and transition between the two. This robot utilizes a combination of surface tension and buoyancy to support its weight and generates differential drag using passive flaps to swim forward and turn. Electrowetting is used to break the water surface and transition into water by reducing the contact angle, and subsequently inducing spontaneous wetting. Finally, several design modifications help the robot overcome surface tension and climb a modest incline to transition back onto land. Our results show that microrobots can demonstrate unique locomotive capabilities by leveraging their small size, mesoscale fabrication methods, and surface effects.
This paper develops a place invariant based deadlock prevention method to obtain an optimal, i.e., maximally permissive, liveness-enforcing Petri net supervisor with a minimal supervisory structure ...that means the minimal number of control places. Maximal permissiveness can be achieved by designing place invariants that make all legal markings reachable while all first-met bad markings unreachable. An integer linear programming problem is formulated to compute all place invariants and its objective function minimizes the number of place invariants, aiming to yield a minimal supervisory structure. Importantly, we develop a technique to greatly improve the efficiency of the proposed method by reducing the number of constraints and variables in the integer linear programming problem under consideration. A number of examples from the literature are used to illustrate the proposed approaches.
The prognosis of the remaining useful life (RUL) of turbofan engine provides an important basis for predictive maintenance and remanufacturing, and plays a major role in reducing failure rate and ...maintenance costs. The main problem of traditional methods based on the single neural network of shallow machine learning is the RUL prognosis based on single feature extraction, and the prediction accuracy is generally not high, a method for predicting RUL based on the combination of one-dimensional convolutional neural networks with full convolutional layer (1-FCLCNN) and long short-term memory (LSTM) is proposed. In this method, LSTM and 1- FCLCNN are adopted to extract temporal and spatial features of FD001 andFD003 datasets generated by turbofan engine respectively. The fusion of these two kinds of features is for the input of the next convolutional neural networks (CNN) to obtain the target RUL. Compared with the currently popular RUL prediction models, the results show that the model proposed has higher prediction accuracy than other models in RUL prediction. The final evaluation index also shows the effectiveness and superiority of the model.
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